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In new wiring, when running a new circuit from the main panel in the basement to multiple rooms upstairs, what is the most efficient way to wire lights in each room so they are 1.) on the same circuit and 2.) controlled by an individual switch in each room?

I'm sure I can run my main 12-2 or 14-2 (I will use 12) to the closest light in this circuit, and continue the power to the next room, but I am not sure what the best method of switching the power in each individual room is. Or if I should run power to each room's switch and then use a new wire to go from each powered switch to the series of each room's lights.

  • Just a note or two...our county codes require 14ga/15a for lights (using 12ga or 20a breakers is illegal). Most fans require 20a brkrs and 12ga cable. Mixing 12ga with 14ga on the same circuit is expressly forbidden. In general, 14ga runs from 15a brkrs and 12ga runs from 20a brkrs. It might be prudent to check local code requirements as they change regularly. – cappy102 Feb 25 '18 at 18:05
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In the old days you could run neutral and ground to all the lights and a single hot throu the switch in each room to the lights in that room. BUT, since 2011 NEC code requires all 3 wires to be available in each switch box even if the neutral is just capped off (this is for remotely controlled switches - smart homes).

Here is a nifty diagram for you (source) light switch diagram

Seems sort of a waste though to be doing it this way now that LED and occupancy (motion/light) sensors are cheap. At minimum, installing a dual switch with one low level LED light fixture is preferable to most people and even Leviton is making inexpensive bluetooth remotes so people can have a switch wherever they put the bed. Some combination of these has greatly improved our family's quality of lighting and now it's unimaginable to have to go back to the single full light on/off scenario.

You asked for efficiency and that can be interpreted differently: do you want to save an hour on the initial installation or make living/working there efficient for years to come?

  • That diagram confuses me because there is no other light getting power. I am assuming that a 12-2 wire is just tied into the 3 wire nuts in the light's receptacle box. to continue power down the row, but what about other lights that will be switched off that one switch? How do you efficiently wire the other lights on that switch AND keep a live wire going to the next room at all times? When I said efficient, I meant the most efficient wire runs, I'm fine with having to have a neutral capped off - I want to take the best wire route to have the least wire waste AND meet all code. – Shumardii Aug 22 '17 at 15:29
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The most 'efficient' method would be to tap off at a switch to feed power to the next switch. The only reason here being that you'd be able to run 12-2 for the entire circuit (assuming single pole switching only with the example given).

Wiring layout

The only true difference in this over running to the light first and creating a switch loop is that you are now required to have a neutral at each switch box - which would then call for 12-3 for the loops.

As well though, this is on the condition of new wiring in presumably new construction - giving you the freedom on locating the wiring. But at the same time, it all comes down to the distance to the fixtures and switches. If you have a light right by the panel and the switch is on the other side of the room, it'd be easier to run the short distance of homerun 12-2 to the light and then a 12-3 loop to the switch rather than the two long distance runs of 12-2. Seeing as 12-2 is nearly half the cost of 12-3 though, you'd be able to essentially run a 2 wire across the room and back for the same as using a single 3 wire. In this case it'd be efficient in only have a single strand of romex between each device, but you'd be paying a little more if you don't have 12-3 on hand - making it less cost efficient.

In short; it depends on the situation based on the locations of everything. Running 12-2 throughout, tapping off to feed other rooms at the switches, is probably the simplest answer to being the most 'efficient'. Simply because you're now required to have access to neutral at each switch.

EDIT: I'll tack this on for extra measure. Your primary hot coming from the panel, regardless of how the wires are ran physically, must tap off such as this to feed each switch in parallel to one another. The the load on each switch is as well included in parallel powered from that particular switch.

Circuit layout for multiple rooms

  • Thanks. I'd like to have that neutral there for certain switches. I know my fan controller switch in the bathroom needs the extra wire. I will have 5 lights in each room and want to include 3 rooms per circuit. Each light is an LED that draws no more than 15W so I don't see any issue with that. Sounds like the best method is to run 12-2 from panel to switch in room 1, then run the same wire to room 2 switch 1 and room 3 switch 1, then come back to room 1 switch 1 and run to room 1 light 1, 2, 3, 4, 5 and repeat each step? – Shumardii Aug 22 '17 at 15:42
  • Also, in each room I have a fan; either ceiling fan or exhaust fan switched in the same box as the light switch. I presume I can just pigtail to switch 2 in each room from the source wire. – Shumardii Aug 22 '17 at 15:43
  • @Shumardii That's right - you're getting it now. I've included another graphic at the bottom of my answer that might help some as well. Your hot source wire (the homerun to the panel) must feed into each switch. – TFK Aug 22 '17 at 16:04
  • So the piece of 12-2 romex that goes from switch to light has the black wire connected to the physical switch leg while the white wire and ground wire are just wired into the pigtail via wirenuts. Then at each light, I use a ~6" pigtail and each wire nut will house 3 wires: source wire, 6" pigtail wire to light, and the feed wire for the next light in that room? Back at the switch, for hot wire I will have source wire, 6" pigtail to switch 1, 6" pigtail wire to switch 2, feed wire for room 2 switch 1. And the same for neutral - just that neutral may be capped off? Or can I omit that? – Shumardii Aug 22 '17 at 16:27
  • That's all correct. You would cap off the neutral, but you still have to have it within the box. – TFK Aug 22 '17 at 16:52
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We can't answer that because it all depends on the physical topology of your particular construction.

The key design issue is to run always-hot and neutral to every switch and lamp location - then add a third switched-hot wire (red typically) between the switch and the lamp(s) it controls, by using 12/3 cable for that segment.

You absolutely can and should use a "tree" topology, branching a spur line wherever that makes sense to do.

At that point, wiring is dog simple: at any splice, reds go to reds, whites go to whites and blacks go to blacks. A lamp taps red and white. A switch taps black and red, unused whites at switches are capped.

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You can easily do this by running your 12-2 between the light fixture boxes themselves. This should be easy inside the attic. Just pick a starting point and drop one back to the breaker.

When you wire your switches, you should drop 14-3 so you get the required neutral to the box.

  • So to keep things simple and maintain low wire waste, I should essentially start at the breaker, run up to the attic, find the very first light and put in some "loops" at each light location in each room all in the same wire run, then return and cut each loop and run a 12-3 from each switch to each light? What is the best method to keep power going to the next room and switch power on and off the the multiple lights in each room. Assume each room has 5 lights, and there are 3 rooms on a circuit. I'm making this too complicated and confusing myself severely. – Shumardii Aug 22 '17 at 15:33
  • So let's say they all have 5 boxes. Pick a single box in that room and bring the circuit into and out of that box and into the next room. Then you'll bring your switch wire into this box. From there, build a circuit from that box to the other 5 in that room – Machavity Aug 22 '17 at 15:35
  • I commented to @TFK below, to keep things clean I believe I should run my source wire to each switch. I think I may have more room in a switch box than my light's box depending on the light fixture... – Shumardii Aug 22 '17 at 15:47
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here's a quick diy sketch (had to post as separate answer because of obscure rules). This uses all 14/2 (3 wire). diy sketch

protected by Community Nov 5 '18 at 2:13

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